The invention relates to x-ray filaments used as the cathode member of an x-ray tube for use in diagnostic and therapeutic radiology machines, for example computerized axial tomography (CAT) scanners. More specifically, the invention is directed to a x-ray filament having a morphology and composition exhibiting ductility and thermal and mechanical shock resistance, and a method of manufacturing the same.
A conventional x-ray tube assembly, which is typically enclosed in an oil-filled protective lead casing to absorb produced heat, comprises a glass envelope containing a cathode member, a rotating disc target which forms the anode, and a rotor. The rotor forms part of a motor assembly that spins the target. A stator is provided external of the x-ray tube proximate to the rotor, overlapping about two-thirds the length thereof. The glass envelope is provided with a window to permit the exit of the x-rays generated by the x-ray tube.
The production of x-rays results from the sequential release, acceleration and abrupt stoppage of electrons generated within a vacuum, in the x-ray tube. In order to release electrons, the cathode member that includes a helical wire filament positioned in a cathode cup, is electrically heated to incandescence by means of the passage of electrical current there through. Subsequently, the released electrons are accelerated by the application of a high voltage of the order of from about ten thousand to several hundred thousands of volts between the cathode and the anode of said x-ray tube. Directionally controlled impingement of the accelerated released electrons upon the rotating target anode causes stoppage of the electrons thereupon at different points upon the anode perimeter and consequent release of x-rays. The high voltages required to operate the x-ray tube are supplied by a transformer, the alternating current being rectified utilizing rectifier tubes or alternatively by barrier-layered rectifiers.
The electrons from which x-rays are generated are provided by the cathode assembly comprising a coiled filament cathode housed in a metallic cup. Heretofore, such wire filaments have been constructed from a potassium-doped tungsten wire, which exhibits excellent structural stability and focusing characteristics at the high operational temperatures required for electron emission and x-ray generation. X-ray tube performance can be affected by the alignment of the filament in the cathode, thus the coiled tungsten filaments were assembled and then aligned in the cathode cup. Once assembled, the filaments were heated to about 2800xc2x0 C. to produce the desired recrystallized microstructure. During this heating, the filaments often sagged, move out of alignment, thus necessitating realignment thereof and repetition of the heat treatment step.
This sagging and alignment problem was addressed in U.S. Pat. Nos. 5,498,185; 5,514,413; and 5,672,085 all issued to B. A. Knudsen et al., and assigned to the assignee of the instant application, the disclosures of which are herein incorporated by reference. These patents, which include detailed descriptions of the prior art x-ray tubes assemblies, are directed to methods of filament alignment and to the provision of a x-ray tube cathode assembly having a solid, one-piece insulator unit associated therewith.
Recrystallized, doped tungsten filaments possesses low ductility at room temperature and the recrystallized cathodes become extremely brittle. Furthermore, such doped tungsten filaments exhibit major deficiencies in applications wherein there is substantial thermal and mechanical shock. The thermal shock can be generated, for example, by rapid thermal cycling of the filament during exposure to a rapid cycling CAT x-ray system. Under these conditions, the filament temperature is raised almost instantaneously to emission temperatures of about 2500xc2x0 C. The high-speed rotation and counter rotation in the gantry of a scanning CAT x-ray system, for example, may generate high mechanical shock, AND such systems becoming an increasingly important application for this type of x-ray tube.
It is known to construct filaments for incandescent lamps from a tungsten-rhenium alloy wherein the rhenium content is in a range from about 3 to about 30 percent by weight (w/o) with the balance comprising tungsten. The addition of rhenium to tungsten and doped-tungsten was recognized as providing the benefit of increasing the ductility, which enhances resistance to thermal and mechanical shock. However, such tungsten-rhenium wire of the prior art generally possessed a fine equiaxial microstructure which detrimentally affected the creep performance of filaments made therefrom and caused sagging of such filament wire when exposed to high temperature. Sagging of filaments due to creep is a recognized cause of misalignment of the cathode wire filament within the cathode cup (discussed above) and is known to result in improper focusing of x-rays emanating from such filament. The service life of a filament member of tungsten-rhenium was generally thought to be limited due to such creep and the resultant creep failure, which would ensue at the high temperatures, and over the time period over which x-ray filaments operate. Accordingly, at high temperatures in the range of above 2300xc2x0 C. (the temperature at which x-ray tubes typically operate), the prior art has recognized the unsuitability of tungsten-rhenium wire due to the significant creep at these temperatures. (ref. H. J. Frost and M. F. Ashby, xe2x80x9cDeformation Mapsxe2x80x94The Plasticity and Creep of Metals and Ceramicsxe2x80x9d, pp. 150-152, Pergamon, 1982.)
The invention provides a heat-treated, recrystallized, potassium-doped tungsten-rhenium filament, suitable for high temperature operation in the cathode assembly of a x-ray tube. The filament comprises enhanced creep life over tungsten and doped-tungsten filaments. The filament, as embodied by the invention, is subjected to a heat-treatment step that defines a recrystallized, generally uniform microstructure having a grain size greater than about 20 microns. The magnitude of the grain size provides enhanced creep strength concomitant with the retention of thermal and shock resistance properties inherent in tungsten-rhenium filaments. In particular, the x-ray filament of the invention comprises rhenium in a range from about 3.0 to about 7.0 weight percent, and potassium in a range from about 30 to about 110 PPM, with tungsten making up the balance of the composition, and possessing interlocked grains of a grain size greater than about 20 microns.
Prior to installation within the cathode cup, the filament of the invention is heat-treated at temperatures in a range from about between 2600xc2x0 C. to about 3230xc2x0 C. and for a time in a range from about 0.1 minutes to about 5 hours, for example in the range from about 3170xc2x0 C. to about 3230xc2x0 C. for a period in a range from about 1.5 to about 3.0 minutes, and alternatively heated in a range from about 2870xc2x0 C. to about 2930xc2x0 C. for a time period of approximately 4 hours, so as to alone or in combination with a drawing schedule, produce a filament having interlocked grains with an average grain size greater than about 20 microns.
The filament, as embodied by the invention, permits the use of tungsten-rhenium wire filaments in x-ray devices, and permits the consequent advantages of tungsten-rhenium filaments to be realized. By addition of potassium doping within certain ranges and further adjusting grain size through heat treatment and/or forming processes to produce grain size in excess of about 20 microns, the benefits of tungsten-rhenium filaments can be obtained in x-ray applications, including but not limited to resistance to at least one of thermal and mechanical shock over known tungsten and doped-tungsten filaments.
More particularly, the invention comprises a x-ray filament adapted for use as the cathode of an x-ray tube, which comprises a coiled wire having a composition comprising rhenium in a range from about 3.0 to about 5.5 weight percent, with the balance being tungsten and being doped with potassium in a range from about 30 to about 110 PPM, and having interlocked grains of a grain size greater than about 20 microns.
In a further aspect of the invention, a method of making a coiled wire filament for use as the cathode of an x-ray tube comprises:
(i) forming a wire member, substantially comprising rhenium in a range from about 4 to about 6 weight percent with the balance being tungsten doped with potassium in a range from about 30 to about 110 PPM, from a rod in a series of one or more drawing passes, each drawing pass reducing the cross sectional area of the wire member;
(ii) the cumulative reduction in cross sectional area of said wire member being at least about 40 per cent;
(iii) forming coils in said wire member; and
(iv) thereafter heat treating said wire member by heating to a temperature in the range from about 2600xc2x0 C. to 3230xc2x0 C. for a time period in a range from about 0.1 minutes to about 5 hours; to produce a filament having an interlocked grain structure of average grain size in excess of about 20 microns.
These and other aspects, advantages and salient features of the invention will become apparent from the following detailed description, which, when taken in conjunction with the annexed drawings, where like parts are designated by like reference characters throughout the drawings, disclose embodiments of the invention.